A variety of microsurgical procedures have been developed which have saved the lives of patients and/or improved the quality of life for patients. Such procedures include organ transfer surgery, reconstructive surgery following the removal of tumors (particularly in the areas of the head and neck), CABG procedures, and reconstructive surgery such as free tissue transfer and the like. Free tissue transfer entails the removal of tissue and/or muscle from one part of the body, along with an associated artery and vein, and the reattachment of the tissue and/or muscle to another part of the body. The artery and vein of the transferred tissue and/or muscle are then anastomosed (that is, connected) to a native artery and vein in order to achieve blood circulation in the transferred tissue and/or muscle.
The success of such transfer lies in obtaining good patency of the anastomosis, and hence good patency in the transferred tissue and/or muscle (sometimes referred to as the flap). The primary complication in microvascular surgery such as free tissue transfer is thrombosis. Unrecognized thrombosis reduces patency in the flap and reduces the probability of salvaging the flap. The window of opportunity for salvage after thrombosis is presently believed to be only about six hours of warm ischemia. It is therefore critical that any vascular thrombosis in a transferred flap be recognized and any resulting ischemia be remedied as soon as possible. While the success rate of the free tissue transfer procedure is quite good, believed to be about 90% on average, failure rates have been reported ranging from 6% to 21%. Even though these are fairly low, any surgical failure can be costly in several ways, and it would of course be highly desirable to reduce the failure rate of this and similar techniques.
A variety of operative and post-operative monitoring techniques are presently used for clinically assessing thrombosis and identifying the resulting ischemia. Electromagnetic flowmetry is a definitive technique to monitor blood flow; but so far this technique has proven too difficult to use in free tissue transfer. Some of the other techniques that have been clinically studied include intravenous fluorescein, transcutaneous oxygen, tissue pH, pulse oximetry, muscle contractility, temperature, photoplethysmography, electrical impedance plethysography, and the like. Unfortunately; these techniques are not useful for monitoring bone flaps or other flaps which are located well below the skin, that is, buried. Newer techniques include surface temperature measurement, pO.sub.2 monitoring, and laser Doppler flowmeters, but these also require the presence of an exposed portion of the flap, and are of no benefit for monitoring bone or buried flaps. Moreover, none of these techniques evaluates the flow of blood at the microvascular anastomoses directly. Thus, no technique has been universally accepted as an adequate monitor of thrombosis.
One assessment technique gaining wider acceptance is the use of an implantable ultrasonic Doppler probe positioned directly on the anastomosed vein and/or the artery. Such a probe includes an implanted piezoelectric transducer carried on a band or sleeve attached directly on the blood vessel of interest. The transducer is used to alternately generate ultrasonic waves and measure backscattering of those waves. Since blood is a very effective backscattering medium, the Doppler shift in the frequency of the backscattered ultrasonic waves yields a precise and accurate measurement of the blood velocity (and, by implication from the cross-sectioned area of the blood vessel, the volume of blood flow) in the vessel of interest. Signals relating to the phasic velocity and mean velocity of the blood can also be obtained. The Doppler probe is thus advantageous over prior techniques in that it permits easy monitoring of vascular patency in even buried flaps, and can be used to monitor patency continuously over a period of days. Careful monitoring of blood flow should provide a sufficiently early warning of thrombosis that the chances of salvaging the flap are significantly increased.
Unfortunately, the use of known ultrasonic Doppler probes and techniques has been subject to a number of drawbacks. The two most significant drawbacks have been the inability to securely place the probe on the blood vessel (perivascularly), and thus ensure proper orientation of the operative surface of the piezoelectric transducer, so as to acquire reliable signals from the transducer; and the difficulty of removing the probe from the vicinity of the blood vessel without the use of anaesthesia, or without performing an additional surgical incision or reopening an old incision.
For example, U.S. Pat. No. 5,289,821 (William M. Swartz, Mar. 1, 1994) discloses a device in which electrically conductive wires carrying an ultrasonic transducer are connected by a silicone adhesive to a biologically inert or absorbable strip, the strip being wrapped about the blood vessel of interest. After three to seven days, when monitoring is complete, an incision is made to detach the wires from the strip, and the transducer removed from the patient by pulling on the wires. While the patent asserts that this new incision is very small, it is difficult to see how a practitioner could withdraw the transducer, wire, and the sealant without causing trauma to the surrounding tissue or, even worse, the anastomosed vessel. To minimize this removal trauma, the practitioner would normally make the incision large enough to permit this withdrawal. However, the enlarged incision would cause unnecessary discomfort to the patient. Moreover, the device lacks structure to maintain orientation of the transducer during introduction to and implantation on the blood vessel, or from breaking off the transducer and/or the wires during removal. Failure to maintain proper orientation of the transducer can lead to false blood flow or velocity readings and as a result cause further surgical intervention with associated patient discomfort and prolongation of the monitoring process.
Other known devices and techniques have their own drawbacks. Accordingly, it would be highly desirable to achieve a device for monitoring blood flow during or after a surgical procedure which can be easily and quickly attached to a blood vessel. It would also be highly desirable that such a probe be firmly attachable to the blood vessel, such that reliable and repetitive monitoring signals be acquired for the period necessary to ensure patency of the surgical procedure, for example, for at least three weeks. It would also be advantageous for such a probe to be easily removable from the patient without entailing the performance of additional invasive procedures; ideally, the probe would be removable at the patient's bedside, with an ease equal to that of removing a conventional drainage catheter. It is also desirable that such a probe be useful with arteries as well as veins, and be useful for monitoring vessels having a range of diameters, particularly, those above 1 mm diameter. Of course, it would be essential to ensure that the orientation of the operative surface of the ultrasonic transducer be fixed, and to reduce or eliminate any potential for detachment of the transducer from the probe or sheath carrying it.